Coating compositions

ABSTRACT

A coating composition which is capable of curing at ambient temperature to form a coating comprises (A) an anhydride-functional polymer containing at least two cyclic carboxylic acid anhydride groups per molecule and (B) an amine-functional polymer and is characterized in that the amine-functional polymer (B) contains at least two secondary amine groups per molecule, the polymer (B) being substantially free from hydroxy and ether oxygen atoms and amine nitrogen atoms in the alpha and beta positions with respect to the amine nitrogen atoms, and the carbon atoms directly bonded to the amine nitrogen atoms not being tertiary carbon atoms.

TECHNICAL FIELD

This invention relates to a coating composition capable of curing atambient temperature.

BACKGROUND ART

U.S. Pat. No. 4,452,948 describes a coating composition suitable for useas a glossy pigmented coating for a rigid substrate, particularly wherehardening of the coating at ambient temperatures is required, forexample as a vehicle refinishing paint. The coating compositioncomprises a hydroxy component having at least two free hydroxyl groupsper molecule and an anhydride component having at least two cycliccarboxylic acid anhydride groups per molecule, at least one of thesecomponents being a film-forming polymer. The composition also includes acatalytically effective amount of amine groups for accelerating thecuring reaction between the hydroxyl groups and the anhydride groups.The amine groups are preferably incorporated in the molecule of thehydroxy component. Tertiary amine groups are preferred although certainsecondary amine groups, for example those containing tertiary alkylgroups such as polymerised units of t-butylaminoethyl methacrylate, canbe used.

European Patent 73022 describes an anhydride-functional polyester andclaims its use as a hardener for compounds having free epoxide, amine orhydroxyl groups. European Patent Application 282184 describes agrease-compatible, plasticiser-extended encapsulant for splicedelectrical or optical cable comprising an anhydride-functionalisedcomposition and a crosslinking agent such as a polyol, polyamine orpolythiol.

European Patent Application 259172 describes a coating compositioncomprising an anhydride polymer containing at least two cycliccarboxylic acid anhydride groups and a polymer containing at least twofunctional groups reactive with anhydride groups. These reactive groupsare selected from hydroxyalkylamino, hydroxyalkoxyalkylamino,hydroxy-substituted acyloxyalkylamino, hydroxy-substitutedpolyacyloxyalkylamino, mercaptoalkylamino and oxazolidino groups. One ofthe polymers comprises a flexible polymer chain selected from polyether,polyester, silicone, diene polymer, hydrogenated diene polymer,polyurethane, polyisobutylene and polyacrylate chains. The functionalgroups characteristic of that polymer are each present as a terminalgroup at the end of a flexible polymer chain. The cured coatings haveincreased resistance to impact and abrasion. The hydroxyalkylaminogroups are preferably tertiary amino groups but can be secondary aminogroups formed for example from the reaction of a primary amino groupwith an epoxide or from the reaction of a carboxy-tipped polymer withN-hydroxyethyl imine.

Disclosure of Invention

A coating composition according to the present invention comprises (A)an anhydride-functional polymer containing at least two cycliccarboxylic acid anhydride groups per molecule and (B) anamine-functional polymer and is characterised in that theamine-functional polymer (B) contains at least two secondary aminegroups per molecule, the polymer (B) being substantially free fromhydroxy and ether oxygen atoms and amino nitrogen atoms in the alpha andbeta positions with respect to the amine nitrogen atoms, and the carbonatoms directly bonded to the amine nitrogen atoms not being tertiarycarbon atoms.

By secondary amine groups we mean groups in which an --NH-group isdirectly bonded to two carbon atoms which themselves are directly bondedonly to carbon or hydrogen atoms. By tertiary carbon atoms we meancarbon atoms which are directly bonded to three other carbon atoms.

The amine-functional polymer (B) is preferably substantially free fromhydroxyl groups. The number of primary amine groups in polymer (B)preferably amounts to no more than 10% of the total number of primaryand secondary amine groups in the said polymer.

The coating compositions of the invention cure by reaction of theanhydride groups of polymer (A) with the secondary amine groups ofpolymer (B) to form amide linkages. The resulting cured polymer withamide crosslinks is more resistant to hydrolysis than the polymers ofU.S. Pat. No. 4,452,948 and European Patent Application 259172, whichcontain ester crosslinks formed by the reaction of anhydride groups withhydroxyl groups and unchanged tertiary amine or hindered secondary aminegroups.

Compositions in which the amine-functional polymer (B) contains onlyprimary amine groups fall outside the scope of the present invention.Such compositions have an unacceptably short pot-life when used ascoating compositions. Secondary amines react considerably more slowlywith carboxylic acid anhydrides than do primary amines, so removing thisdisadvantage. Secondary amines in which the nitrogen atom is directlybonded to a tertiary carbon atom react too slowly with carboxylic acidanhydrides to give an acceptably short curing time for a coatingcomposition.

Amides which contain a hydroxy or ether oxygen atom (as inhydroxyalkoxy, hydroxy-substituted acyloxy or hydroxy-substitutedpolyacyloxy groups) or amino nitrogen atom in the alpha or beta positionwith respect to the amide nitrogen atom exhibit a poorer resistance tohydrolysis than those which do not contain such groups. The former typeof grouping is therefore undesirable in a cured coating composition.

The amine-functional polymer (B) can for example be a polyether,polyester, polyamide, acrylic polymer, silicone, diene polymer,hydrogenated diene polymer, polyurethane or polyisobutylene. Thesecondary amine groups are preferably present as terminal groups at theends of a polymer chain; for best impact resistance they are preferablypresent at the ends of a flexible polymer chain as described in EuropeanPatent Application 259172.

Polymers tipped with secondary amine groups are not generally availablecommercially but can be prepared from polymers containing primary aminegroups by an alkylation reaction. The primary amine-functional polymercan for example be reacted with an alkyl halide, especially a bromide orchloride such as methyl bromide or ethyl bromide, or with an alkylsulphate or sulphonate ester such as dimethyl sulphate or methylp-toluenesulphonate. Such an alkylation reaction is preferably carriedout in organic solution in the presence of a base, for example atertiary amine such as triethylamine or an alkali metal salt of a weakacid such as sodium or potassium carbonate.

An alternative alkylation reaction is a Michael-type addition reactionbetween a primary amine and an alpha, beta-unsaturated carbonyl orcarboxy compound. The reaction is preferably carried out at 50° to 200°C. in a polar organic solvent. For example, a primary amine-functionalpolymer can be reacted with an ester of an alpha, beta-unsaturatedcarboxylic acid ##STR1## where Z represents a radical of the polymermolecule; R¹, R² and R³ each independently represent hydrogen or amonovalent organic group linked through a carbon atom, for example analkyl group; and R⁴ represents an alkyl or aryl group which can besubstituted. Examples of suitable unsaturated carboxy compounds areesters of acrylic or methacrylic acid or diesters of maleic, fumaric oritaconic acid, for example methyl acrylate, ethyl acrylate, butylacrylate, methyl methacrylate or dimethyl fumarate.

The primary amine-functional polymer can alternatively be reacted withan unsaturated ketone ##STR2## where Z, R¹, R² and R³ are each asdefined above and R⁵ represents an alkyl or aryl group which can besubstituted. An example of a suitable unsaturated ketone is mesityloxide (CH₃)₂ C═CHCOCH₃.

Examples of primary amine-functional polymers suitable for reaction withthe unsaturated carbonyl or carboxy compound include amine-tippedpolyethers. Examples which are commercially available includepoly(tetramethylene oxide) of various molecular weights from 750 to2,200 tipped with 3-aminopropyl groups, polypropylene oxide of molecularweights from 400 to 3,000 tipped with aminoalkyl groups and lowmolecular weight polyetheramines such as4,7,10-trioxatridecane-1,13-diamine. Aminoalkyl-tippedpolydiorganosiloxanes are also commercially available. Other primaryamine-functional polymers can be prepared from carboxylicacid-functional polymers, for example copolymers of acrylic ormethacrylic acid, by reaction with aziridine.

The Michael addition reaction can also be used to introduce amine groupsinto an ethylenically unsaturated polymer. For example, a polyesterhaving ethylenically unsaturated end groups can be formed by reacting aminor proportion of an unsaturated monocarboxylic acid such as acrylicor methacrylic acid with the glycol and polycarboxylic acid or anhydridereagents used to prepare the polyester. The polyester containing theacrylic or methacrylic residue can be reacted, preferably in ahydrocarbon solvent such as toluene, with a primary amine to formsecondary amine groups. Alternatively, an ethylenically unsaturatedpolyester can be formed using units derived from maleic acid oranhydride, itaconic acid or fumaric acid and can be reacted with aprimary amine to introduce pendent secondary amine groups spaced alongthe polyester molecule.

An amine-functional polyamide can be produced by the reaction of adicarboxylic acid with an excess of a diamine. One preferred type ofdicarboxylic acid is a fatty acid dimer. The diamine can be a secondaryamine, in which case a secondary amine-tipped polyamide is produceddirectly, or a primary amine, in which case a primary amine-tippedpolyamide is formed which can be alkylated as described above.

A carboxy-functional polymer, for example a carboxy-tipped polyester,can be reacted with an N-alkyl ethylenimine to introduce --COOCH₂ CH₂NHR⁶ groups, where R⁶ represents alkyl. A polyurethane having isocyanateend groups can be reacted with a protected primary amine such as theketimine of aminoethanol to introduce aminoalkyl, for exampleaminoethyl, groups bonded to the polymer through a urethane linkage. Theamine groups can be alkylated to secondary amine groups as describedabove.

Primary amines can react with ketones which have at least one hydrogenatom in an alpha position to the carbonyl group to form enamines thatare vinyl-substituted secondary amines:

    R.sup.7 CHCOCR.sup.8 R.sup.9 +R.sup.10 NH.sub.2 →R.sup.7 C=C(NHR.sup.10)CR.sup.8 R.sup.9

where R⁷ is a monovalent organic group linked through a carbon atom tothe remainder of the molecule and may be alkyl but is preferably anunsaturated group such as acryl or aryl capable of stabilising theenamine by conjugation; R⁸ and R¹⁰ are monovalent organic groups linkedthrough a carbon atom to the remainder of the molecule, for examplealkyl or aryl; and R⁹ is hydrogen or has the structure defined for R⁸and R¹⁰. This reaction can be utilised to prepare secondaryamine-functional polymers suitable for use as component B. In onealternative, a primary amine-functional polymer of any of the typeshereinbefore mentioned is reacted with a keto-functional compound suchas ethyl acetoacetate to convert the primary amine groups to secondaryamine groups. In another alternative, a keto-functional polymer isreacted with a primary amine to introduce secondary amine groups intothe polymer.

The anhydride-functional polymer (A) is preferably an addition polymerof an ethylenically unsaturated cyclic anhydride, for example maleic,itaconic or citraconic anhydride. The anhydride-functional polymer isusually a copolymer with one or more ethylenically unsaturatedcomonomers. Preferred copolymers contain 10 to 50% by weight maleic oritaconic anhydride units. The anhydride-functional polymer may contain avinyl comonomer such as styrene or a substituted styrene, vinyl chlorideor vinyl acetate. If the coating is to be used as a decorative top coatit may be preferred to use such a vinyl comonomer at a molar ratio of atleast 1:1 to the anhydride monomer, as described in U.S. Pat. No.4,798,745. The anhydride-functional polymer preferably contains units ofone or more esters of acrylic or methacrylic acid, for example butylacrylate or methacrylate, methyl methacrylate or acrylate, ethylacrylate or methacrylate or propyl, n-hexyl, isopropyl, t-butyl,2-ethylhexyl, cyclohexyl, 3,3,5-trimethylcyclohexyl or isobornylacrylate or methacrylate. The molecular weight of theanhydride-functional polymer is preferably in the range 1,000 to 50,000.

The anhydride component (A) can alternatively be an anhydride adduct ofa diene polymer such as maleinised polybutadiene or a maleinisedcopolymer of butadiene, for example a butadiene/styrene copolymer.Maleinised polymers of this type have high reactivity with theamine-functional polymer (B) but are less preferred than the ethyleniccopolymers described above because they form cured coatings which arenot as hard. An anhydride adduct of an ethylenically unsaturated fattyacid ester, for example a styrene/allyl alcohol copolymer esterifiedwith an ethylenically unsaturated fatty acid and maleinised, can also beused. Terpene/maleic anhydride copolymer resins are a furtheralternative.

Alternative anhydride-containing polymers can be formed fromhydroxy-containing polymers, for example copolymers of hydroxyethylacrylate or hydroxyethyl methacrylate or styrene/allyl alcoholcopolymers, by reaction with a tricarboxylic compound capable ofintroducing anhydride groups, for example as described in EuropeanPatent Application 259172. A further alternative type of polymercontaining anhydride groups is an adduct of trimellitic anhydride and apolyol, for example as described in European Patent Application 134691.The polymer containing anhydride groups can alternatively be formed bythe reaction of a polymer containing thiol groups with an ethylenicallyunsaturated cyclic carboxylic acid anhydride such as maleic anhydride oritaconic anhydride. The polymer containing thiol groups is preferablythiol-tipped; it can for example be a multi-limbed telechelic polymerformed by reaction of a corresponding hydroxy-tipped polymer withmercaptoacetic acid.

The amounts of anhydride-functional polymer (A) and amine-functionalpolymer (B) in the coating composition of the invention are preferablysuch as to provide a ratio of equivalents of anhydride groups toequivalents of amine groups in the range 1:3 to 3:1, most preferably1:1.2 to 1.2:1.

The coating composition of the invention is preferably an organicsolvent-based composition. The solvent is chosen so that theanhydride-functional polymer (A) and the amine-functional polymer (B)are soluble to the desired extent and are compatible with one another insolution. Organic solvents which can be used are for example aromatichydrocarbons such as xylene, toluene or trimethylbenzene, esters such asbutyl acetate, ethoxyethyl acetate or methoxypropyl acetate, ketonessuch as methyl isobutyl ketone or methyl isoamyl ketone and halogenatedhydrocarbons such as methylene chloride, chloroform or trichloroethane.

The coating composition of the invention is generally a two-pack coatingin which the anhydride-functional polymer (A) and the amine-functionalpolymer (B) are stored separately and are mixed shortly before use. Thecoating can be applied to the substrate by spray, for exampleconventional airless spray or twin-feed spray in which the polymers (A)and (B) are not mixed until the spray head, or by roller or brush or bydipping or flow coating. The coating composition can be applied to awide variety of substrates, particularly to rigid substrates such asmetal, wood, glass or plastics. The compositions can be applied overmost commercially sold primers. The coating is generally capable ofcuring on the substrate to a tough, tack-free film within 24 hours atambient temperatures, for example 10° to 40° C. Curing may be carriedout at temperatures above ambient, for example up to 100° C. or 150° C.,for shorter times if this is more convenient, for example when coatingunder factory conditions.

The coating composition can contain one or more additional ingredients.It will usually contain one or more pigments which can be opaque ortranslucent pigments or metallic flake pigments and optionally one ormore fillers. Alternatively, the coating composition may be unpigmented,for example for use as the top transparent clear coat in a "clear onbase" coating system used as an automotive paint, particularly a paintfor vehicle refinishing. The coating composition can contain one or moreplasticisers, antioxidants, UV stabilisers or flow control agents,including additives for imparting thixotropy or sag resistance orpigment orientation. In two-pack compositions such additionalingredients are preferably included in the amine-functional polymercomponent of the paint. The coating composition can if desired contain atertiary amine catalyst although such tertiary amine catalyst isgenerally not necessary. If a tertiary amine catalyst is used it can bea separate compound such as N,N-dimethylcocoamine, or tertiary aminegroups can be incorporated in the molecule of the amine-functionalpolymer (B).

The invention is illustrated by the following Examples.

EXAMPLE 1 (a) Preparation of Anhydride Copolymer

650 g styrene, 150 g methyl methacrylate, 200 g itaconic anhydride and40 g azo bis-2,2'-(2-methylbutyronitrile) initiator were mixed and runinto refluxing butyl acetate (1,000 g) over 4 hours. When addition wascomplete, reflux was maintained for a further 2 hours to provide apolymer solution of solids content 50% by weight, anhydride equivalentweight 560 and number average molecular weight Mn 4,500.

(b) Preparation of Amine-Functional Polymer

85 g bis(aminopropyl)-tipped poly(tetramethylene oxide) of molecularweight 750 and 20 g ethyl acrylate were dissolved in 200 g chloroformwith 5 g triethylamine and 2 g hydroquinone. The solution was refluxedfor three hours and allowed to stand overnight. Volatile materials wereremoved by rotary evaporation, leaving a clear yellow oil which was apolyether tipped with --NHCH₂ CH₂ COOC₂ H₅ groups.

(c) Coating Composition

5.6 g of the anhydride copolymer solution produced in (a) above wasmixed with 2.6 g of the amine-functional polymer produced in (b) above,diluted with 1 g methyl ethyl ketone. The coating composition producedwas cast on glass plates at 75 microns wet-film thickness and wasallowed to cure at room temperature (about 20° C.) for three days. Itssolvent resistance was then tested by rubbing with a cloth soaked inmethyl ethyl ketone (MEK). The solvent resistance was 40 MEK doublerubs.

EXAMPLE 2

The process of Example 1b was repeated using 40 g methyl methacrylate inplace of the ethyl acrylate. The polymer produced was used in place ofthe polymer of Example 1b in preparing a coating composition accordingto Example 1c. The cured film had a solvent resistance of 70 MEK doublerubs.

EXAMPLE 3

The process of Example 1b was repeated using 19.6 g mesityl oxide inplace of the ethyl acrylate. The polymer produced was used in place ofthe polymer of Example 1b in a coating composition prepared according toExample 1c. The solvent resistance of the cured coating was 100 MEKdouble rubs.

EXAMPLE 4 (a) Preparation of amine-functional polymer

85 g of the bis(aminopropyl) polyether used in Example 1b was mixed with22 g ethyl bromide and 50 g triethylamine in 200 g ethanol. The reactionmixture was refluxed for three hours and was allowed to stand overnight.The product was diluted with 200 g xylene and was filtered to remove theprecipitated triethylamine hydrobromide. The filtrate was then subjectedto rotary evaporation to remove volatile materials, yielding a clearyellow oil which was a polyether tipped with N-ethylaminopropyl groups.

(b) Coating composition

5.6 g of the anhydride copolymer solution produced in Example 1a wasmixed with 2.25 g of the amine-functional polymer produced in Example4a, diluted with 1 g methyl ethyl ketone. The coating composition wasapplied to glass plates and cured as described in Example 1. The curedcoating had a solvent resistance of 100 MEK double rubs.

What we claim is:
 1. A coating composition, comprising:(A) Ananhydride-functional polymer containing at least two cyclic carboxylicacid anhydride groups per molecule, and (B) an amine-functional polymer,wherein the amine-functional polymer (B) contains at least two secondaryamine groups per molecule, and wherein in said amine-functional polymer(B) the alpha and beta positions with respect to the amine nitrogenatoms are substantially free from hydroxy and ether oxygen atoms andfrom amino nitrogen atoms, and wherein in said amine-functional polymer(B) the carbon atoms directly bonded to the amine nitrogen atoms are nottertiary carbon atoms.
 2. A coating composition according to claim 1, inwhich the amine-functional polymer (B) is substantially free fromhydroxyl groups.
 3. A coating composition according to claim 1, in whichthe number of primary amine groups in the amine-functional polymer (B)amounts to no more than 10% of the total number of primary and secondaryamine groups in said polymer (B).
 4. A coating composition according toclaim 1, in which the amine-functional polymer (B) is a polyether,polyester, polyamide, acrylic polymer, silicone, diene polymer,hydrogenated diene polymer, polyurethane or polyisobutylene and thesecondary amine groups are present as terminal groups at the ends of apolymer chain.
 5. A coating composition according to claim 1, in whichthe anhydride-functional polymer (A) is an addition polymer of anethylenically unsaturated cyclic anhydride.
 6. A coating compositionaccording to claim 5, in which the anhydride-functional polymer (A) is acopolymer containing 10 to 50% by weight of maleic or itaconic anhydrideunits.
 7. A coating composition according to claim 1, in which theamount of anhydride-functional polymer (A) and amine-functional polymer(B) are such as to provide a ratio of equivalents of anhydride groups toequivalents of amine groups of 1:1.2 to 1.2:1.